1. Field of the Invention
The present invention relates generally to a method and apparatus for measuring formation parameters by transmitting and receiving electromagnetic signals by means disposed in recesses of a tubular housing member. More particularly, the present invention is related to downhole logging tools which use propagating electromagnetic waves to perform measurements of formation or borehole parameters.
2. Description of the Background
It is desirable for many reasons to transmit electrical signals through the Earth as a propagating medium, and to receive the signals at a location spaced from the transmitter. Such a signal propagation system is, for example, used both for the determination of various parameters associated with the propagating medium and for communication purposes. These systems are often used in the investigation of the environment surrounding a borehole, and in particular, the surrounding formations. Various types of borehole logging systems are available to perform these investigations. A class of these systems utilize electromagnetic field phenomena to obtain data from the environments surrounding the borehole. One type of prior art logging is electrode logging which utilizes an electric field in the surrounding formation to produce a measure of the conductivity of the formation. A conductive mud is necessary for proper use of this system, thus rendering the system inoperative with oil based muds. Inductive logging is another type of prior art electromagnetic logging which uses a time-varying magnetic field in the formation to produce a secondary current flow in the formation. The secondary current flow sets up a second magnetic field which induces current in receiving coils positioned in the borehole, the induced current in the receiving coil or coils being proportional to the secondary current flow in the formation and thus is directly proportional to the conductivity or inversely proportional to the resistivity of the surrounding formation. Electromagnetic wave propagation for investigating the environment around a borehole is the subject of the present invention.
An electromagnetic logging system of the wave propagation type is disclosed in Gouilloud et al., U.S. Pat. No. 3,551,797, which is incorporated herein by reference. This patent discloses a wire line system having a transmitter and receivers for measuring formation parameters, and utilizing phase comparison and amplitude. However, the Gouilloud wire line system is not particularly useful in a measuring while drilling (herein after, "MWD") configuration. The Gouilloud patent discloses a nonconductive sonde of insufficient strength to operate in a drill string characterized by massive steel and more particularly drill collars in the vicinity of the drill bit and measurement apparatus. U.S. Pat. Nos. 4,107,597 and 4,185,238 also show electromagnetic wave propagation systems for use in wire line apparatus. U.S. Pat. No. 4,107,597, for example, describes the wire line sonde as being constructed of a non-conductive material which is customary in such devices in order to accommodate the use of electromagnetic transmitting and receiving apparatus.
U.S. Pat. No. 3,079,550 shows an induction logging system for measuring similar formation parameters, utilizing lower frequencies and requiring a conductive mud in the borehole.
An improvement over each of the above-referenced electromagnetic logging systems is described in U.S. patent application Ser. No. 036,170, filed on Apr. 6, 1987 and assigned to the Assignee of the present application, now U.S. Pat. No. 4,875,247. This present application is an improvement over the methods and apparatus described in the said application No. 036,170.
One of the major problems associated with downhole logging tools, whether they be wireline or MWD, is that of overcoming borehole effects. Another failure mode associated with electromagnetic wave propagation is that of damage to the antennas.
The primary object of the present invention is to provide a new and improved electromagnetic wave propagation tool which has a greatly enhanced immunity to borehole effects.
It is yet another object of the invention to reduce the susceptibility of the electromagnetic wave propagating antennas to damage while operating downhole.